Lactams polymerization, activated monomer mechanism

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Polymerizations initiated by strong bases (R-, IIO, RO-) and tertiary amines (which are poor nucleophiles) proceed at much faster rates than do polymerizations initiated hy primary amines. Also, unlike the latter, where each polymer chain contains one initiator fragment (i.e., RNH—), these polymerizations do not result in incorporation of the initiator into the polymer chain. Polymerization proceeds by an activated monomer mechanism similar to that in the anionic polymerization of lactams. The reacting monomer is the NCA anion XLIV... 

Polymerization of oxirane (and of its derivatives) by the mechanism of activated monomer is so far exclusively cationic and can be represented by schemes (27) and (28) of Chap. 4. In contrast to the ring-opening polymerization of lactams, both the classical and the activated monomer mechanisms are operating in this case. Conditions can be found where one or the other mechanism predominates [339]. 

In the polymerization of unsubstituted lactams, propagation proceeds mainly by the activated monomer mechanism ... 

The polymerization occurs by the activated monomer mechanism, which supposes a two-step mechanism involving the acylation of the lactam anion (NaL) by the AT-acyllactam end-group followed by a fast proton-exchange with the monomer. In bulk polymerization, the preformed AT-acyllactams or their precursors (so-called CIs) are introduced into the system in order to avoid the slow initiation step due to the absence of AT-acyllactam groups at the beginning of polymerization ... 

The mechanism of the cationic polymerization of lactams formally resembles the activated monomer mechanism, better known for the anionic process. In this media-... 

Looking at these reactions, a specularity between the anionic and the cationic mechanism is observable. Different from conventional ionic polymerizations, the growth center in cationic and anionic lactam polymerizations is always neutral, and it is the lactam monomer that carries the ionic charge. Such polymerizations follow what is sometimes referred to as activated monomer mechanism, since the reaaive species are lactam cations/anions, thus activated monomers. However, in order to avoid misunderstandings, it must be well dear that this definition has nothing to do with the term activated... 

The interpretation of the mechanism of anionic lactam polymerization based on the conventional scheme (ionic active centre with approaching monomer) could not exhaustively explain all the observed effects. Agreement could only be obtained when the acido-basic properties of lactams and polyamides had been respected. The equilibrium... 

Lactam polymerization with anionically activated monomer has its counterpart in the cationic processes of lactam polymerization. This type of mechanism has also been observed recently in some polymerizations of oxygen-containing heterocycles (see Chap. 4, Sect. 2.3)... 

So far, not many systems are known to meet the conditions of the growth mechanism with activated monomer. Polymerizations of oxirane, chlo-romethyloxirane [145, 146] and of lactams [147] belong to this class. The existence of an electroneutral propagating macromolecule and of an activated monomer results in reduced probability of back-biting reactions and in easier preparation of macromers [145] by means of exchange reactions. 

There are essentially two mechanisms of chain growth in the cationic polymerization of lactams. Either the cationically activated monomer reacts with neutral growth center or the neutral monomer molecule reacts with cationic active centers located at the end of the growing chain [214) ... 

While other strong bases, such as liAlH4 and Buli, behave similarly to NaH in the deprotonation of monomer, yielding the classical anionic mechatusm previously illustrated, lithium dialkoxyaluminum hydrides behave differently. The aruon obtained by the reaction of CL with lithium dialkoxyaluminum hydrides is less nucleophilic than the classical lactamate anions."" "" A new mechanism has been suggested " for this type of polymerization where different active species are involved. 

The polymerization proceeds differently in the case of lactams, with the nucleophilic species being carried alternatively by the monomer and the growing chains. For the monomer to be inserted at the chain end, it needs to be activated through proton abstraction and charge transfer before it can add onto the end-standing carbonyl group such a mechanism is called activated monomer polymerization, and it is an unusual process in chain polymerization ... 

The mechanism of the anionic polymerization of lactams has been studied mainly for CL-the most easily accessible (and industrially produced) lactam. The basis for the mechanism of anionic polymerization of lactams is fixed [5, 12-14], and differs from conventional anionic polymerization. The process involves addition of the monomeric lactam anion (the so-called anionically activated monomer ) onto a nonionic growth center located at the end of the growing chain. Thus, lactam anion 2 is formed by the reachon of a proper nucleophile with lactam 1 (Equation 7.1) ... 

The Frunze etal. mechanism has much in common with the alkali lactamolytic mechanism of Champetier and Sekiguchi, except for the formation of the above-shown complex. Frunze et al. also believe that probably a single mechanism exists for the anionic polymerization of lactams that they describe as ion coordinative Tht contributions of various mechanisms via ion pairs or via free ions depend upon the nature of the alkali metal counterion and upon their capacity to coordinate with electron-donating compounds (activator and monomer). The growth of ion pairs may mainly be expected from a lithium counterion, while growth by free anions may be expected from potassium or cesium. 

Because of their free electron pairs or electron shell vacancies heteroatoms are particularly susceptible to attack by catalyst. Since heteroatom-containing groups react both by polycondensation and polyaddition mechanisms, they are especially readily catalyzed. The polymerization of rings with heteroatoms (lactams, lactones, trioxane, etc.) can also be easily initiated. On the same basis, however, deactivation of the chain can also occur frequently in these substances therefore, only low degrees of polymerization can be obtained. On the other hand, the activation of cycloalkanes in polymerization reactions proceeds with difficulty. The following discussion will therefore concentrate on the polymerization of rings and monomers with multiple bonds. 

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